1. Technical Field
The present invention relates to a technique for suppressing a noise component for a signal representing a sound (hereinafter, referred to as “sound signal”) in which a desired signal component (target sound component) and a noise component are mixed.
2. Background Art
Conventionally, various techniques for suppressing a noise component of a sound signal (or emphasizing a signal component) have been proposed. For example, in Non-Patent Document 1 or Patent Document 1, a spectrum subtraction method for subtracting an estimated spectrum of a noise component (hereinafter, referred to as “estimation noise spectrum) from a spectrum of a sound signal is disclosed.    [Non-Patent Document 1] Ephraim Y., Malah D., “Speech enhancement using a minimum-mean square error short-time spectral amplitude estimator”, DECEMBER 1984, IEEE TRANSACTIONS ON ACOUSTICS, SPEECH, AND SIGNAL PROCESSING, VOL. 32, NO. 6, PP. 1109-1121    [Patent Document 1] JP-A-2003-131689
However, in the technique of Non-Patent Document 1 or Patent Document 1, a noise component may not be completely removed. A noise component remaining in an interval in which the strength of a signal component is low is remarkably perceived by a listener. In particular, there is a problem in that a noise component irregularly remaining on a time axis and a frequency axis is perceived as strident musical noise (birdie noise). A level of suppressing an estimation noise spectrum from a spectrum of a sound signal needs to be increased in a situation where a signal to noise ratio is low, but the musical noise is remarkably perceived as the suppression level of the estimation noise spectrum is increased.
In view of the above situation, an object of the present invention is to make it difficult to perceive a noise component (particularly, musical noise).
A noise suppressing apparatus related to one aspect of the present invention is provided for addressing the above problem. The inventive noise suppressing apparatus suppresses a noise component of a sound signal which contains the noise component and a signal component. The noise suppressing apparatus comprises: a frequency analyzing section that divides the sound signal into a plurality of frames such that adjacent frames overlap with each other along a time axis, and that computes a first spectrum of each frame; a noise suppressing section that suppresses a noise component of the first spectrum so as to provide a second spectrum of each frame in which the noise component is suppressed; a frequency specifying section that specifies a frequency of a noise component of each frame; a phase controlling section that varies a phase of the noise component corresponding to the specified frequency in the second spectrum by a different variation amount each frame; and a signal synthesizing section that combines the frames after the second spectrum of each frame is processed by the phase controlling section, such that adjacent frames overlap with each other along the time axis so as to output the sound signal.
According to the above configuration, the clearness of the noise component is reduced by varying a phase of the noise component by a different variation amount in each frame. Accordingly, this can make it difficult to perceive a noise component (for example, musical noise) as compared with a configuration in which a sound signal after suppression by a noise suppressing section is directly output.
In case that a signal component is specified and then the remaining component is specified as a noise component, the frequency specifying section includes a section that specifies a frequency of a signal component. Moreover, the frequency specifying section uses any information to specify the frequency of the signal component. For example, the frequency of the noise component can be specified on the basis of the first spectrum computed in the frequency analyzing section or the second spectrum after processing by the noise suppressing section. The frequency of the noise component can be specified on the basis of a spectrum obtained by means separate from the frequency analyzing section or the noise suppressing section.
The noise suppressing apparatus related to a preferred aspect of the present invention includes a variation amount setting section that sets a different variation amount according to a random number generated for each frame. The phase controlling section varies the phase of the noise component corresponding to the specified frequency by the different variation amount set by the variation amount setting section for each frame. According to the above aspect, the clearness of musical noise can be effectively reduced since phase variation amounts of the frames are set according to random numbers.
According to a preferred aspect, the phase controlling section varies the phase of the noise component corresponding to the specified frequency provided that the specified frequency falls in a predetermined frequency range of the second spectrum. The predetermined frequency range is set, for example, to include a frequency capable of being easily perceived by a listener. According to the above aspect, there is advantageous in that an amount of processing by the phase controlling section is reduced in comparison with a configuration in which a phase is controlled for noise component frequencies over all frequency range. There can be adopted a configuration in which the phase controlling section selectively controls only a phase of a frequency belonging to a predetermined frequency range among noise component frequencies specified in the frequency specifying section, or a configuration in which the frequency specifying section specifies only a frequency belonging to a predetermined frequency range.
The noise suppressing apparatus related to the present invention is realized with hardware (an electronic circuit) of a DSP (Digital Signal Processor) or the like dedicated to suppress a noise component, and is also realized with a cooperation of a general-purpose arithmetic processing unit of a CPU (Central Processing Unit) or the like and a program. A computer program related to one aspect of the present invention is executable by a computer for suppressing a noise component of a sound signal which contains the noise component and a signal component. The computer program comprises: a frequency analyzing process of dividing the sound signal into a plurality of frames such that adjacent frames overlap with each other along a time axis, and computing first spectrum of each frame; a noise suppressing process of suppressing a noise component of the first spectrum so as to provide second spectrum of each frame in which the noise component is suppressed; a frequency specifying process of specifying a frequency of a noise component of each frame; a phase controlling process of varying a phase of the noise component corresponding to the specified frequency in the second spectrum by a different variation amount each frame; and a signal synthesizing process of combining the frames after the second spectrum of each frame is processed by the phase controlling section, such that adjacent frames overlap with each other along the time axis so as to output the sound signal.
Moreover, the present invention is provided as a method for suppressing a noise component. The noise suppressing method related to one aspect of the present invention suppresses a noise component of a sound signal which contains the noise component and a signal component. The method comprises: a frequency analyzing process of dividing the sound signal into a plurality of frames such that adjacent frames overlap with each other along a time axis, and computing first spectrum of each frame; a noise suppressing process of suppressing a noise component of the first spectrum so as to provide second spectrum of each frame in which the noise component is suppressed; a frequency specifying process of specifying a frequency of a noise component of each frame; a phase controlling process of varying a phase of the noise component corresponding to the specified frequency in the second spectrum by a different variation amount each frame; and a signal synthesizing process of combining the frames after the second spectrum of each frame is processed by the phase controlling section, such that adjacent frames overlap with each other along the time axis so as to output the sound signal.